The present invention relates to electronic adaptor circuits which convert combined stop and turn signal lights of a trailer to operate in response to the independent stop and turn signal lights on a towing vehicle. Such devices are well known. Examples of trailer light adaptors are disclosed in U.S. Pat. Nos. 4,005,313; 4,057,310; 4,270,115; 4,405,190; 4,718,853; 4,842,524 and 4,939,503. Some of these patents disclose a circuit for adapting a three lamp system of a towing vehicle to a two lamp system of the trailer. Other patents such as U.S. Pat. Nos. 3,849,664; 3,883,845; 3,970,860; 4,006,453; 4,064,413; 4,325,052; 4,751,431; 5,030,938; 5,212,469 and 5,701,116 disclose trailer light converter systems having certain advantages. For example, U.S. Pat. No. 4,006,453 discloses a converter circuit which adapts a two lamp trailer signal light system to a three lamp towing vehicle system. In this device and others the power to drive the trailer signal lights is provided by the towing vehicle stop and turn signal lights which also serve as ground for the logic circuit. Such a design requires a complex diode isolation network. However, if the towing vehicle lights are missing or defective the logic of the converter will be defeated. Furthermore, the requisite diode isolation network in combination with output transistors, significantly reduce the power output to the trailer signal lights. In addition, many current vehicle lighting circuits employ sensors to detect defective signal lights. Connecting such signal lights to provide power to trailer signal lights causes a significant drain on the towing vehicle lights which can cause disruption of the defective lamp sensors and faulty indications.